For example, as an electrode portion of an electric resistance welder for welding a projection bolt to a counterpart member, there is known an electrode disclosed in Patent Literature (for example, see Patent Literature 1). Note that, an iron projection bolt 1 (hereinafter, simply referred to as bolt) is, as illustrated in FIG. 8, a shaft-shaped component comprising a shaft portion 2 having an outer peripheral surface in which a male thread is formed, a flange portion 3 integrally formed with the shaft portion 2, and a welding projection 4 formed on a flange surface on a side opposite to the shaft portion 2. Further, the counterpart member is a plate-like component such as a steel plate.
FIG. 9 illustrates a schematic structure of an electrode portion 101 disclosed in Patent Literature 1. As illustrated in FIG. 9, in the electrode portion 101, a guide tube 103 made of an insulating material is inserted inside a substantially cylindrical, conductive electrode main body 102. Further, the electrode portion 101 is provided with a receiving hole 104, which is opened at an end surface of the electrode main body 102 in a state in which the receiving hole 104 is communicated to an inside of the guide tube 103. An inner peripheral surface of the receiving hole 104 in a part on an opening leading end side is formed of the conductive electrode main body 102.
On the other hand, inside the guide tube 103, which forms the inner peripheral surface of the receiving hole 104 in a part except for the part on the opening leading end side, a conductive rod 106 having a magnet 105 embedded therein is housed under a state in which the position thereof is restricted in an advancing direction (lower direction in FIG. 9). A coil spring 108 is provided in a tensioned state between the rod 106 and a conductive metal washer 107 arranged at the deepest portion of the receiving hole 104. In this manner, the rod 106 is arranged inside the receiving hole 104 in a freely advanceable/retractable state.
In the electrode portion 101, when the shaft portion 2 of the bolt 1 is inserted into the receiving hole 104, the shaft portion 2 of the bolt 1 is attracted by the magnet 105 embedded in the rod 106, and thus the shaft portion 2 is attached to a leading end portion of the rod 106. After the attachment, when the electrode portion 101 is pressurized to the counterpart member (not shown), the flange portion 3 is brought into intimate contact with the leading end surface of the electrode main body 102. Thus, there is formed an energizing path formed of the metal washer 107, the coil spring 108, the rod 106, the flange portion 3 of the bolt 1, and the leading end surface of the electrode mainbody 102. With energization through this energizing path, the state that the bolt 1 is present and the shaft portion 2 is normally inserted into the receiving hole 104 is detected.
As described above, after the state that the bolt 1 is present and the shaft portion 2 is normally inserted into the receiving hole 104 of the electrode portion 101 is detected, with the detection signal as a trigger, the electrode main body 102 is advanced, and thus the rod 106 is retracted against the elastic force of the coil spring 108. As a result, the flange portion 3 of the bolt 1 abuts against the end surface of the electrode main body 102. Under this state, a welding current is applied, thereby welding the projection 4 of the flange portion 3 to the counterpart member (not shown).
Note that, when the bolt 1 is absent or the bolt 1 is present but the shaft portion 2 thereof is abnormally inserted into the receiving hole 104, the above-mentioned energizing path is not formed. As a result, a non-energizing state is established, and the state that the bolt 1 is absent or the bolt 1 is present but the shaft portion 2 thereof is abnormally inserted into the receiving hole 104 can be detected. With this detection signal as a trigger, the electrode main body 102 is not advanced and the welding operation is stopped.